The present disclosure relates generally to shoulder prostheses, and more particularly to shoulder prostheses configured for use in shoulders having glenoid vault erosion or defects.
A typical shoulder or glenohumeral joint is formed in a human body where the humerus 10 movably contacts the scapula 12 as shown in FIG. 1. The scapula 12 includes a glenoid fossa 14 that forms a socket against which the head of the humerus 10 articulates. At this socket, the scapula 12 includes cartilage 16 that facilitates such articulation. Beneath the cartilage is subchondral bone 18 that forms a wall of a glenoid vault 20 that defines a cavity which contains cancellous bone 22. The subchondral bone 18 that forms the glenoid vault 20 defines a glenoid rim 21 at a periphery of the glenoid vault that is attached to the cartilage 16 (see FIG. 1). During the lifetime of a patient, the glenoid fossa 14 may become worn, especially at its posterior and/or superior portions thereby causing severe shoulder pain and limiting the range of motion of the patient's shoulder joint. To alleviate such pain and increase the patient's range of motion, a shoulder arthroplasty may be performed.
Shoulder arthroplasty often involves surgical replacement of the glenoid fossa with a conventional glenoid prosthesis such as the one disclosed in U.S. Pat. No. 6,911,047, the disclosure of which is herein incorporated by reference. The glenoid prosthesis, when implanted, provides a new laterally-facing bearing surface, which may be concave or convex, for articulation with a complementary bearing surface of a natural or prosthetic humeral head. Such conventional glenoid prosthesis is typically formed from UHMW polyethylene, titanium, or cobalt chrome and includes bone anchor(s) such as peg(s), screw(s), post(s), or a keel extending from a back side of the device opposite its bearing surface. So configured, the back side of the prosthesis is typically secured against subchondral bone of the glenoid vault while the bone anchor(s) may extend into the cavity of the glenoid vault whereby it may become anchored to cancellous bone located within the glenoid vault.
Shoulder prostheses such as those described above are very effective. During typical movement of the shoulder joint after arthroplasty, however, the humeral head rotates and slides against the glenoid surface. Glenoid components, however, are typically spherical in shape. Accordingly, the translation of the humeral head in the shoulder joint after arthroplasty can result in edge loading of the prosthesis assembly. Edge loading can result in limited motion, instability, and accelerated wear.
Some attempts have been made to reduce the problems associated with edge loading. Some prosthesis assemblies introduce a mismatch in the diameter of the head and the diameter of the glenoid component. This approach accommodates translation of the humeral head but does not eliminate edge loading. Additionally, the resulting loss of optimal coverage of the humeral head results in increased instability. Another approach that has been used is to use varying diameters in the glenoid component. This approach also fails to eliminate edge loading and further results in reduced stability.
What is needed therefore is an improved prosthesis assembly for use in patients requiring shoulder arthroplasty. An improved prosthesis assembly which reduces edge loading without increasing instability of the shoulder joint is also needed.